In eyes with prolonged visual deprivation induced by the abnormally large optical defects such as aberrations, the actual visual performance after precise correction of remaining aberrations is significantly poorer than that predicted from optical theory and that measured in normal eyes. This unexplained vision loss suggests that the degraded image quality received by the eyes alters neural processing of images formed on the retina, which plays an important role in determining perceived visual quality. We hypothesize that the post-correction functional measurements on a given patient are biased by long-term neural adaptation to the poor retinal image quality that the patient may have progressively experienced before correction. We will test this hypothesis using a corneal disease, keratoconus as a model of long-term visual adaptation. The visual system of this unique patient group developed normally but, during adulthood, has gradually experienced severely degraded image quality by the large magnitude of aberrations for a prolonged period of time. The proposed project implements the latest tools and advances in human optics research to investigate (1) the mechanisms underlying long-term neural adaptation to degraded optical quality of the eye and (2) neural plasticity resulting from improved optics and/or visual training paradigms. We will use two innovative advanced correction tools: an adaptive optics vision simulator and a customized scleral lens for short-term and long-term precise aberration correction, respectively. Aim 1 is designed to investigate the mechanisms that underlie long-term neural adaptation to the optically degraded retinal image quality and their impact on neural processing of image quality by (1.1) testing the hypothesis that the neural system is capable to compensate for losses in image quality due to the ocular aberration through long-term adaptation to phase spectra using broadband stimuli, acuity letters and natural images (1.2) characterizing long-term adaptation induced-changes in the key properties of basic spatial vision mechanisms using narrow band visual stimuli i.e. gratings and (1.3) examining the effects of long-term neural adaptation on the two eyes being integrated with regard to the monocular functions including visual acuity, contrast perception at and above threshold before and after aberration correction. Aim 2 will assess the extent to which plasticity that occurs during long-term adaptation is reversible and what mechanistic changes underlie this reversal once aberration-free image quality is achieved in KC eyes. (2.1) We will first quantify the time course of passive neural re-adaptation to improved ocular optics achieved by wearing customized aberration correcting scleral lens daily. We will also apply different visual training paradigms based on (2.2) narrow (single spatial frequency gratings) and (2.3) broad (natural images) band visual stimuli to differentiate different mechanisms of neural plasticity and to test the hypothesis that visual performance can further be improved by the visual training. Binocular transfer of the monocular neural manipulation through visual training effects will also be examined.